Sputter resistive cold cathode for low pressure gas discharge device



Dec. 23, 1969 K. G. HERN-Qv'xs'r SPUTTER RESISTIVE COLD CATHODE FOR LOWPRESSURE GAS DISCHARGE DEVICE Filed Sept. l2, 1967 /NVENTR www.

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ATTORNEY United States Patent O US. Cl. 313--211 11 Claims ABSTRACT FTHE DISCLOSURE There is disclosed a cold cathode having an extremelylong life which is particularly adapted for use in a gas dischargedevice, such as a gas laser, in which the gas pressure is no greaterthan ten torrs. The cold cathode comprises a porous layer of a normallynon-emissive sputter resistant insulator, such as alumina, having anelectrode embedded therein and a small amount of introduced alkali metalwhich is adsorbed by the surface of the insulator layer. The coldcathode is located in the terminating region of the discharge deviceenclosure to take advantage of the cataphoresis effect.

This invention relates to gas discharge devices and, more particularly,to an improved cold cathode for a low pressure gas discharge device,such as a gas laser, which by being sputter resistive provides a verylong life for such a low pressure gas discharge device.

The use of cold cathodes for gas discharge devices, such as the familiarneon tube, which operate at gas pressures of ten torrs or more is old inthe art. These cold cathodes usually comprise mainly an alkali earthcompound, such as barium oxide or calcium oxide. Irl some cases a traceof an alkali metal is present for the purpose of lowering the workfunction of electron emission from the cathode. At gas pressures of tentorrs or more the amount of sputteringy which takes place from such analkali earth cold cathode is relatively small and such a cold cathodewill operate satisfactorily for an extended period of time.

A gas laser, as known in the art, consists of means for discharging asuitable gas, such as carbon dioxide, a noble gas, or a mixture of noblegases, at suitable pressure within an optical resonant cavity. The gaspressure suitable for gas lasers, depending upon the particular gasbeing utilized, ranges from a few tenths of a torr to several torrs. Inany case, the suitable gas pressure for gas lasers is below ten torrs,the minimum gas pressure normally employed in conventional gas dischargedevices utilizing cold cathodes.

At low pressures (pressures below ten torrs) the mean free path of gasions becomes quite large resulting in the gas ions which bombard thecold cathode having much higher velocities and kinetic energies than isthe case where the gas pressure is relatively high (ten torrs or more).

It has been found that when conventional cold cathodes composedessentially of alkali earth compounds are utilized in low pressure(below ten torrs) gas discharge devices, such as a gas laser, the largekinetic energy of the bombarding gas ions causes excessive sputtering ofthe cold cathode to take place. This results in such a cold cathode whenutilized in a low pressure gas discharge device having a very shortlife, such as onehalf hour, for instance.

It is therefore an object of the present invention to provide animproved cold cathode for a gas discharge device operating at a gaspressure below ten torrs which has a very long life time.

ICC

It is a more specic object of the present invention to provide a coldcathode which is substantially resistant to sputtering when employed ina gas discharge tube having a gas pressure below ten torrs.

Briefly, in accordance with the present invention, the cathode electrodeis embedded in a porous mass of any given substantially sputterresistant and normally nonemissive insulator which is chemically inertto a given alkali metal and such given alkali metal is made present tothis mass of porous insulator so that a substantial portion of thesurface of the mass is normally covered by some adsorbed given alkalimetal. In addition, the cold cathode is inserted within a terminatingregion of the gas discharge enclosure which is connected to theremaining region of the gas discharge enclosure solely throughconstricting means comprising a small opening. The size of the openingis suliciently small to result in the temperature at the opening duringthe discharge of gas therethrough being maintained high enough tosubstantially prevent any of the surface of that portion of theenclosure defining the opening from being covered by the given alkalimetal.

The normally non-emissive insulator can be made into an effectivesputter resistive cathode by the presence of adsorbed alkali metal. Thisis true because the alkali metal has a low electron emission workfunction; the adsorbed alkali metal at least to a certain extent islocated within the pores of the mass of insulator, and, most important,the region of the insulator surface under the influence of each particleof adsorbed alkali metal acts as a conducting surface. If contiguousregions which are conductive are in overlapping relationship, which isthe case when there is sufficient adsorbed alkali metal, an effectiveconductor is formed between the embedded electrode and the outer surfaceof the insulator mass.

Another feature of the present invention is that by locating the cathodein a terminating region of the enclosure, any deactivation of thecathode due to a loss of adsorbed alkali metal of the surface thereofduring operation is continuously replaced by the process ofcataphoresis. Also, loss of alkali metal caused by unwanted overheatingof the cathode is temporary. More particularly, most of the alkali metalwill return to the large effective surface of the cathode after it hascooled down. This is because the alkali metal adheres more strongly tothe empty insulator surface than to itself, thus preventing droplets toremain at other parts of the tube. Therefore, damage to the cathode byoverheating is selfhealing, so that after a recuperation period of aboutan hour, normal operation may be resumed.

These and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription taken together with the accompanying drawing in which:

FIG. 1 illustrates an embodiment of a gas discharge laser incorporatingone embodiment of the improved cold cathode of the present invention;

FIG. 2 illustrates another embodiment of the improved cold cathode ofthe present invention; and

FIG. 3 illustrates still another embodiment of the irnproved coldcathode of the present invention.

Referring to FIG. l, there is shown an embodiment 0f the presentinvention which is particularly suitable for use as the gas dischargetube in a gas laser. In particular, the discharge tube comprises tube100, which has a diameter of a few millimeters, coupled at one end torst enlarged region 102 and coupled at the other end to second enlargedregion 104. First enlarged region 102 is attached to optical window 106out at Brewsters angle with respect to the axis of tube 100. Similarly,second enlarged region 104 is attached to optical window 10S also cut atBrewsters angle with respect to the axis of tube 100.

Coupled to first enlarged region 102 by coupling tube 110 is coldcathode 112. Coupled to the second enlarged region 104 by a couplingtube 114 is anode 116. As shown, anode 116 comprises electrode 11S whichis electrically connected to the outside of the gas discharge enclosureby conductors 120 and 122.

Cold cathode 112 comprises a helical electrode 124, which may be made oftungsten, for instance, embedded in a layer of porous insulator 126,such as alumina (A1203) composed of a plurality of particles each .ofwhich particles may have the size of the order of five microns which asshown is attached to the side wall and distal end of the portion of thegas discharge enclosure forming cold cathode 112. Conductor 128 attachedto the end of helical electrode 124 provides electrical connection ofcold cathode 112 to the outside of the gas discharge enclosure. A smallamount of an alkali metal, such as potassium is introduced into cathode112 from a reservoir through a tube 130 which is then sealed off asshown. A ceramic insert 132, located as shown in FIG. l, is providedwith a small opening 134 which having a diameter in the order of onehundred to one hundred fifty mils provides the sole connection betweencoupling tube 110 and cold cathode 112, so as to form a constrictingmeans. The entire gas discharge laser tube is filled with discharge gas,such as a mixture of helium and neon, for instance, at a gas pressure ofconsiderably less than ten torr.

vWhen conductors 122 are connected to a suitable point of positivepotential (not shown) and conductor 128 is connected to a suitable pointof negative potential (not shown) a gas discharge will take place fromthe surface of layer 126, through opening 134, coupling tube 110, firstenlarged region 102, tube 100, second enlarged region 104, coupling tube114, to electrode 11S of anode 116. This will result in the productionof light within tube 100. If, as known in the laser art, the gasdischarge laser tube is placed within an optical resonant cavitycomprising a first and second parallel mirror surfaces (not shown) thefirst of which is outside of Vand in cooperative relationship with thelight passing through window 106 and the second of which is outside ofand in cooperative relationship with the light passing through window108, lasing action will occur and coherent light will be generated.

It has been found that the operation of cold cathode 112. depends upon acertain amount of the alkali metal inserted inside the enclosure of thegas discharge laser tube being adsorbed by porous layer 126 of alumina,which has a high affinity therefor. Although not all of the'surface ofporous layer 126 is covered by adsorbed alkali metal, still some of thealkali metal is adsorbed within the pores of layer 126. Further, it hasbeen found that the single outer shell electron of each adsorbed alkalimetal atom spends a portion of its time within the surrounding surfacemolecules of insulator within its immediate vicinity. This results in asmall region of the surface .of insulator layer 126 in the vicinity ofand under the inuence of each adsorbed alkali metal atom effectivelyacting as a conductor, rather than as an insulator. Therefore, if, as isthe case, there are sufficient number of adsorbed atoms of alkali metalto cause the respective regions under the influence of each of theseadsorbed atoms to overlap, the entire surface of the insulator will actas a conductor. Since the insulator is porous, the effective surfacethereof, to which alkali metal atoms are adsorbed, includes the tortuousmicroscopic paths through the myriads of pores in insulator layer 126.Thus a conductive path electrically connects embedded electrode 124 tothe gas ions impinging on layer 126 of cold cathode 112. However, sincelayer 126 is composed of a sputter resistive insulator, such as alumina,negligible sputtering of this cold cathode material takes place duringthe operation of the gas discharge laser tube.

For example, it was found in practice that the cold cathode of thepresent invention, when employed in a gas discharge laser tube, had notdeteriorated by sputtering or otherwise after two thousand hours ofcontinuous operation of the gas discharge laser tube. The particular gasdischarge laser tube which has been so operated had a quartz tube 100having a three millimeter bore diameter and an effective length ofcentimeters. This gas discharge laser tube was filled with a mixture ofhelium and neon gas and the alkali metal activator for the cold cathodewas potassium.

Another feature of the present invention, as mentioned earlier, is thatalthough cold cathode 112 becomes deactivated when operated considerablybeyond its normal limits to overheat badly, causing an excessive amountof alkali metal to evaporate and settle on the colder envelope surface,this deactivation is only temporary. This is true because the alkalimetal which has settled on the envelope eventually evaporates and mostof it will end up condensing on alumina layer 126 which has a greateraffinity for alumina than any other part of the gas discharge lasertube. Thus any damage to cold cathode 112 is self-healing and after arecuperation period of perhaps an hour, normal operation may be resumed.

Also, any alkali metal lost from the cathode region during operation dueto heating and sputtering effects is continuously replaced due to theeffect of cataphoresis. The cataphoresis effect comes about because thealkali metal atoms are easily ionized in the discharge and due to theirpositive charge drawn back to the negative cathode.

For cataphoresis to be effective it is necessary that cold cathode 112be located in a terminating region of the gas discharge laser tube. Theterm terminating region, as used herein, means that there is no otherregion of the enclosure coupled to the terminating region except that inwhich anode 118 is incorporated and through which the gas dischargetakes place. In other words, in FIG. l, if there were another additionalportion of the gas discharge laser tube enclosure directly to the rightof cold cathode 112 (into which gas could diffuse but in which nodischarge would take place), which is not the case, cold cathode 112would not be located in a terminating region of the enclosure. However,as is the actual case, where cold cathode 112 is coupled to theremaining region of the enclosure solely by coupling tube 110, coldcathode 112 is located within a terminating region of the enclosure.

FIGS. 2 and 3 show alternative forms which the cold cathode of thepresent invention may take.

In FIG. 2, cold cathode 212 incorporates tungsten mesh electrode 224which is embedded in porous alumina layer 226 which is attached to thewalls of that portion of' the enclosure of the gas discharge laser tubedefining the region of cold cathode 212. Connecting electrode 224 to theexterior is conductor 228. In FIG 2, discharge tube 200, which replacesdischarge tube in FIG. 1, is coaxially located as shown with respect tocold cathode 212 and is coupled thereto solely by small opening 234 incylindrical ceramic insert 232. The right end of tube 200 is terminatedin a Brewster angle Window (not shown) and the left end of tube 200,which is terminated in another Brewster angle Window (not shown), iscoupled to region 235 in which is located an anode (not shown). Further,the desired alkali metal is inserted into cold cathode 212 from areservoir through opening 230 prior to the sealing thereof.

In FIG. 3, cold cathode 312 utilizes electrodes 324 consisting of aplurality of tungsten wires embedded in a mass of alumina 326. As showneach of the plurality of embedded tungsten wires has one end thereof,which may have a diameter of ten mils, directly exposed to the gaswithin the gas discharge laser tube. The other end of the plurality oftungsten wires, as shown, are connected to conductor 328 which leads tothe outside. Cold cathode 312 is coupled to the remainder of the gasdischarge tube solely by small opening 334 in ceramic insert 332. Thedesired alkali metal is inserted into cold cathode 312 from a reservoirthrough opening 330 prior to the sealing thereof. The cold cathodeembodiment shown in FIG. 3 is particularly useful for use in pulsedlasers, such as an argon pulse laser, where the exposed ends ofelectrodes 324 permit very high peak discharge currents (in the order of100 amperes) during each pulse.

Although only certain preferred embodiments of the present inventionhave been described in detail herein, it is not intended that theinvention be restricted thereto, but that it be limited only by the truespirit and scope of the appended claims.

What is claimed is:

1. In a gas discharge device comprising an enclosure having a gascapable of electric discharge contained therein, a cathode and an anodein spaced relationship with respect to each other positioned within saidenclosure, first means for electrically connecting said cathode to onepoint outside of said enclosure, and second means for electricallyconnecting said anode to another point outside of said enclosure; theimprovement wherein said enclosure incorporates constricting means fordividing said enclosure into a terminating region and a remaining regionwhich regions are coupled to each other solely by a small opening of agiven area, said anode being located wholly within said remaining regionand said cathode being located wholly within said terminating region,wherein said cathode comprises a porous mass of a given substantiallysputter resistant insulator located within said terminating region, saidgiven insulator being chemically substantially inert to the presence ofa given alkali metal, said mass having an embedded electrode distributedtherewithin which electrode is in contact with said first means, andsaid given alkali metal being present within said terminating region,whereby at least a substantial portion of the surface of said mass isnormally covered by adsorbed given alkali metal, and wherein the size ofsaid given area is sufiiciently small to result in the temperature atsaid opening during the discharge of said gas between said cathode andsaid anode through said opening being maintained high enough tosubstantially prevent any of the surface of that portion of saidenclosure delining said opening from being covered by said given alkalimetal.

2. The device defined in claim 1, wherein said given insulator isalumina.

3. The device defined in claim 1, wherein said mass is in the form of alayer in contact with the inner wall surface of that portion of saidenclosure defining said terminating region.

4. The device defined in claim 1, wherein said embedded electrodecomprises at least one emerged portion which is exposed to the gaswithin said terminating region.

5. The device defined in claim 1, wherein said embedded electrodecomprises a plurality of spaced wires each of which has solely one endthereof emerging from said mass and exposed to the gas within saidterminating region.

6. The device defined in claim 5, wherein each of said wires has adiameter in the order of ten mils.

7. The device defined in claim 1, wherein said constricting meanscomprises a ceramic insert in said enclosure.

8. The device defined in claim 7, wherein said small opening comprises ahole in said ceramic insert having a diameter in the order of onehundred to one hundred fty mils.

9. The device defined in claim 1, wherein said mass is composed of aplurality of particles each of which particles has a size in the orderof iive microns.

10. The device defined in claim 1, wherein the pressure of the gaswithin said enclosure is less than ten torrs.

11. The device defined in claim 1, wherein said gas discharge devicecomprises a gas laser.

References Cited UNITED STATES PATENTS 2,087,735 7/1937 Pirani et al.313-211 X 2,121,589 6/1938 Espe 313-346 2,131,204 9/1938 Waldschmidt313--346 X JAMES W. LAWRENCE, Primary Examiner R. F. HOSSFELD, AssistantExaminer U.S. Cl. XR.

